Wireless power transmission is a technique of wirelessly supplying power to household appliances or electric cars rather than using a power line which is an existing cable. This technique is advantageous in that a device which needs to be supplied power can be charged wirelessly without having to connect the device to a power outlet using a power cable. Thus, research related thereto has been actively conducted.
Wireless power transmission techniques are largely divided into a magnetic induction method, a magnetic resonance method, and a microwave method. The microwave method is a technique of transmitting power by emitting super-high-frequency electromagnetic waves such as microwaves via an antenna. When microwave method is used, long-distance wireless power transmission may be performed but a safety problem caused by electromagnetic waves should be considered. The magnetic induction method is a technique using magnetic inductive coupling between adjacent coils, wherein the distance between two power transmission/reception coils is several centimeters or less and transmission efficiency depends greatly on the arrangement of the two coils. The magnetic resonance method is a technique of transmitting non-radial magnetic-field energy between two resonators spaced from each other through resonant coupling, wherein wireless power transmission may be performed when the distance between power transmission/reception coils is about 1 to 2 m. The magnetic resonance method is more advantageous than the magnetic induction method, in that it is relatively flexible in the arrangement of two coils and a range of wireless charging may be extended using relay coils.
However, when a magnetic field generated by a wireless power transmitter is relayed to a wireless power receiver through relay coils, a flux sum of some relay coils is small due to characteristics of values K and Q thereof and thus charging may not be successfully performed.
In this connection, the invention disclosed in Korean laid-open patent application No. 2012-0040779 concerns a wireless power transmission device which transmits or receives a power signal according to the magnetic resonance method. The wireless power transmission device includes abase coil and a plurality of relay coils, wherein the number of turns of the plurality of relay coils is greater than the number of turns of the base coil.
However, the invention in Korean laid-open patent application No. 2012-0040779 employs the plurality of same relay coils but does not disclose a problem that power cannot be transmitted at a position on a relay coil system or the efficiency of power transmission is very low at this position and a solution thereto.
The invention disclosed in Korean patent No. 1118471 concerns wireless power transmission according to the magnetic induction method, wherein transmission and reception coils are formed of two types of conductive lines.
However, the transmission and reception coils disclosed in the invention in Korean patent No. 1118471 are not relay coils, and a problem that power cannot be transmitted at a position on a relay coil system or the efficiency of power transmission is very low at this position and a solution thereto are not disclosed in this invention.
The invention disclosed in Japanese laid-open patent application No. 2012-0075304 concerns a relay device for magnetic-resonance wireless power transmission, wherein a plurality of relay coils are arranged in a direction of a plane. This invention is mainly directed to improving relaying efficiency.
However, the invention in Japanese laid-open patent application No. 2012-0075304 does not disclose a problem that power cannot be transmitted at a position on a relay coil system or the efficiency of power transmission is very low at this position and a solution thereto.